Hurricanes, Other Vortices Seize
Energyvia "Hostile Takeovers"Research Could Lead to Better Understanding of
Typhoons, Oceanic Flows

For decades, scientists who study hurricanes, whirlpools
and other large fluid vortices have puzzled over precisely
how these vast swirling masses of gas or liquid sustain
themselves. How do they acquire the energy to keep moving?
The most common theory sounded like it was lifted from Wall
Street: The large vortices collect power as smaller
vortices merge and combine their assets, in the same way
that small companies join forces to create a
mega-corporation.

But researchers from The Johns Hopkins University and Los
Alamos National Laboratory now believe the better model is
a much different business tactic: the hostile takeover.
Working with theoretical analysis, computer simulations and
lab experiments, the team has concluded that large fluid
vortices raid their smaller neighbors in an energy grab and
then leave their depleted victims either to wither away or
to renew their resources by draining still smaller
vortices.

Shiyi Chen
Photo by Will Kirk

The findings were published in the March 3 issue of the
journal Physical Review Letters. "This discovery is
important because it could lead to a better understanding
of how hurricanes and large ocean eddies form," said
Shiyi Chen, an
author of the paper. "It should also help us to create
better computer models to make more accurate predictions
about these conditions."

Chen is a professor in the
Department of Mechanical
Engineering at Johns Hopkins, where he occupies the
Alonzo G. Decker Jr. Chair in Engineering and Science. He
supervised the computer simulations in this
two-and-a-half-year research project.

The team looked at large energetic vortex structures that
form in irregular or turbulent two-dimensional flows of gas
or liquid. Common examples are the Red Spot on Jupiter and
hurricanes or typhoons on Earth. The researchers wanted to
figure out how energy is transferred from smaller vortices
to these large-scale circulation patterns. The basic
phenomenon, called "inverse energy cascade," was predicted
almost 40 years ago by pioneering turbulence theorist
Robert H. Kraichnan. However, the dynamical mechanism
underlying the inverse cascade has remained obscure. Does
it occur, as some scientists suggested, through a merger of
small vortices to form a new larger one?

Gregory Eyink
Photo by Will Kirk

"We went into this with an open mind, but we found that the
popular idea of mergers was not correct," said
Gregory
Eyink, a Johns Hopkins professor of
applied mathematics and
statistics and currently the 2006 Ulam Scholar at Los
Alamos Laboratory's Center for Nonlinear Studies. He served
as the primary theorist in the project and was an author of
the journal article. "We found that such mergers are very
rare."

He said the energy transfer actually occurs through a
process described as a "thinning mechanism."

"You have a large vortex spinning around, with a smaller
one inside," Eyink said. "The large vortex has a shearing
effect on the smaller one, like cake batter being stirred.
The large-scale vortex acts like a giant mixer, stretching
and thinning out the smaller one, transferring its energy
into the larger vortex. The large-scale vortex actually
acts like a vampire, sucking the energy out of the smaller
one."

This phenomenon sustains a steady-state inverse energy
cascade. "We end up with a group of large predator vortices
preying on smaller ones, which in turn prey on smaller ones
still, forming a food-chain of vortices," Eyink said.

Johns Hopkins researchers used these banks of
computer servers to produce simulations for their research
into how large fluid vortices, such as hurricanes, acquire
energy to sustain themselves. From left are Minping Wan, a
graduate student in the Department of Mechanical
Engineering; Gregory Eyink, a professor in the Department
of Applied Mathematics and Statistics; and Shiyi Chen, a
professor in the Department of Mechanical
Engineering.Photo by Will Kirk

Through computer modeling at Johns Hopkins and laboratory
experiments at Los Alamos on thin salt-water layers, the
scientists were able to observe the physical processes and
measure the energy transfer. This confirmed their theory
that an energy transfer by stretching of small-scale
vortices is what sustains large-scale vortices.

"This is the first time a quantitative connection has been
made between the process of vortex-thinning and inverse
energy cascade," said Robert Ecke, director of the Center
for Nonlinear Studies at Los Alamos, an author of the
journal article and supervisor of the lab experiments.

The team's research was supported by grants from the
National Science Foundation and the U.S. Department of
Energy. Co-authors include Michael Rivera of the Los Alamos
Materials Science and Technology Division; and Minping Wan
and Zuoili Xiao, both graduate students in the Department
of Mechanical Engineering at Johns Hopkins.

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